The goal of this Phase I study is to determine a safe dose of IB-367 in adults with CF. IB-367 represents a new type of treatment approach to control lung infections. IB-367 belongs to a class of compounds called antimicrobial peptides (small amino acid groups which fight microorganisms). Antimicrobial peptides are found naturally in plants, animals and humans and are considered part of the normal host-defense response (the body's ability to protect against germs and foreign substances). High concentrations of salt in the airways of people with CF may decrease the effectiveness of the body's natural antimicrobial peptides. IB-367 does not appear to be affected by salt concentrations. In the lab IB-367 has been shown to have a killing effect against many organisms, including Pseudomonas aeruginosa and staphylococcus aureus. For these reasons, researchers believe IB-367 may be useful in treating CF lung infections.